The science of health and the study of ways of its preservation, particularly by widespread education and promotion of cleanliness. It plays a vital role in helping to limit the spread of infectious diseases, particularly in developing countries.

The development of hygiene and hygienic practices has been fundamental in preventing infections through cleanliness, and has also been very important in helping patients recover from ailments. On a personal level, it involves washing hands and the body with clean water. Domestic hygiene demands having a clean house environment, with occupational hygiene meaning that workers have a clean workplace. The word itself is derived from Hygieia, the Greek goddess of health and sanitation, who was known as Salus in the Roman Empire. As a word, it first appeared in French as hygiaine in 1597, and Salmon in 1671 noted that there were three speculative parts in medicine: physiology, hygiene, and pathology.

Knowledge about hygiene and cleanliness and the need for it was clearly evident in ancient times with references to it in a number of ancient Hindu texts such as the Hausmriti and the Vishnu Purana. Indeed bathing is one of the daily duties of Hindus, with the custom of washing in the Ganges River at Benares and elsewhere having great symbolic importance. Washing was also practiced by large numbers of ancient peoples with the Romans constructing large public bath houses, while wealthy Romans had their own private bath houses. These often involved a range of pools, some with hot water, and some with cool water, while there were also facilities for massage with oil or scraping the skin with a metal “strigil.” The water was kept fresh by use of the aqueduct. The most impressive of the baths in the Roman Empire were the Baths of Caracalla, in Rome, built from 216 c.e., to accommodate 1,600 bathers. In the New Testament of the Bible, the concept of baptism involves total immersion in water to cleanse one of sins, with Jesus also being involved in the washing of the feet, a practice that has continued into modern times in the Christian Church on Maundy Thursday (the last Thursday before Easter), with the Pope washing the feet of the subdeacons. The Koran prescribes a number of customs surrounding good hygiene, and Muslims wash their faces under running water, and their feet and hands at mosques.

As well as personal cleanliness, the cleaning of wounds has been found to be essential in helping them heal quickly. In Anglo-Saxon times in Britain and some parts of Europe, there was a custom of using honey to treat wounds, but by medieval times, it would found that the wound would heal much faster if washed in clean water.

Although it is now regarded as a healthy and hygienic practice to wash regularly, with many people having a shower at least once a day, this has not always been the case, and was not possible in much of the world where people did not have ready access to fresh water. Although people living near the sea and rivers were able to bathe easily, with the increase in population around the world, and the growth of cities, increasingly more people had to rely on water which was often not fresh. This led to outbreaks of disease such as cholera.

During Elizabethan times in England, Queen Elizabeth I (reigned 1558–1603) and some other wealthy people took up bathing in seawater which was regarded as more hygienic by many people. To do this, bathing boxes were constructed, with the box moved into the seawater, and the person able to bathe inside the box. However there were some people who claimed that washing would remove a protective layer of skin

With increasing interest in personal hygiene, and the growth of cities with consequent overcrowding, and also problems with removal of human waste, the hygiene of buildings became important during Victorian England. In November 1871, Edward, the Prince of Wales (later Edward VII) fell ill at Londesborough Lodge, Scarborough, after having contracted typhoid fever. After huge public concern about what had happened, a medical doctor, William Henry Corfield (1843–1903) attached to University College London, writing to The Times newspaper was able to narrow the problem down to poor house sanitation. This led to new design standards being introduced for public buildings, and the doctor concerned wrote 32 books and treatises on aspects of hygiene in buildings.

Most primary schools teach children to observe hygienic practices including washing one’s hands after going to the lavatory.

In the late 19th century, and the early 20th century, there was an increasing emphasis on workplace hygiene. Hospitals and public buildings, especially schools, were regularly cleaned with disinfectant. However, many factories and mines continued to have people working in unhygienic conditions. The English writer D. H. Lawrence (1885–1930) wrote often of his father returning from the coal mine covered in coal dust. This led to regular demands for the provision of facilities at mines and in factories to allow workers to wash, and have ample and clean lavatories. These basic requirements were gradually introduced from the 1930s, although some mines and factories in less developed countries often have people working in unhygienic conditions, even now.

Nowadays, most primary schools around the world teach children to observe hygienic practices. This involves, at its most basic, washing one’s hands after going to the lavatory, or touching animals, washing the body and hair frequently, and washing of hands before eating. To this is added oral hygiene, with regular brushing of teeth, frequently washing hands, wrists and faces, and the regular washing of clothes and the wearing of clean clothes, as well as living in a clean environment. They are also taught to wash wounds, and not to share cutlery, as well as wash plates after meals and throw out uneaten food.